The present invention provides a method and apparatus for oscillation damping of a wind turbine or wind turbine tower.

Background

At the relevant operating heights of a wind turbine tower or nacelle, winds can be persistent and wind speeds can be elevated. After all, this forms the very reason for siting a turbine at its particular location: namely sites are chosen for their reliable wind, and thereby for effective wind power generation. But windy conditions can present risks to wind turbine structures under certain conditions. For example, at certain wind speeds, a tower, with or without a nacelle at its top, may exhibit so-called oscillation. This is thought to be caused by a phenomenon known as vortex-shedding. This effect may occur predominantly although not exclusively in elongate objects. Under certain conditions, the vortex-shedding effect can cause tower oscillations of high magnitude, comparable to a resonance effect on the tower.

It is thought that a wind turbine tower with no nacelle at its top, e.g. a tower under construction, may be more susceptible to vortex-shedding induced resonant oscillation than an equivalent tower with a nacelle at its top. This may be due to the mass effect of the nacelle reducing the tendency of the tower to oscillate, perhaps by changing the resonant frequency of the tower. In some instances, a nacelle may include an oscillation damping apparatus, such as shown in EP3048295A1 , which may be activated during a period of oscillation in order to limit the oscillations and to bring the wind turbine back under control. For preventing the vortex-shedding effect, it has also been suggested to apply helical strakes to a wind turbine tower to reduce oscillations, as shown in e.g. US4180369. It has also been suggested to apply helical strakes to the exterior of a wind turbine tower under construction, see e.g. EP1881 195 or EP2851490. In some conditions, small strakes may be ineffective. In some conditions, e.g. if a wind turbine is de-powered so that an oscillation damping device may be inactive, or if its natural frequency is altered perhaps by changing the nacelle’s mass, such as during component servicing, then oscillation may nevertheless occur.

Oscillation prevention measures are known, as already mentioned. But once oscillation has started, it can be difficult to interrupt it or bring it to an end, especially if no oscillation damping arrangement is installed or if an oscillation-damping apparatus is depowered or ineffective for any reason. This situation may arise in particular in a tower during its l construction. The present invention is aimed at preventing, reducing, interrupting or stopping wind-induced oscillations at a wind turbine tower.

The need to cease or delay construction operations at a wind turbine when wind speeds increase can significantly increase the cost of installing a turbine or wind park. Longer installation operations due to interruptions or delays or rescheduling as a result of momentary or persistent wind speeds in excess of safe construction conditions can thus adversely affect the cost profile of a project. It is proposed to mitigate this problem by providing a method and equipment for preventing, interrupting or reducing wind induced oscillations at a wind turbine or wind turbine tower, or wind turbine tower under construction.

Summary of the invention

Accordingly, a method is proposed according to which, a local wind shadow is created in the vicinity of a wind turbine tower or segment thereof. This is achieved by holding a wind shield upwind of a wind turbine tower, or wind turbine tower under construction. The shield may be held sufficiently proximate to the tower to reduce the local wind at the tower and thereby to reduce the vortex-shedding effect which may cause oscillation or which may be causing oscillation. Preferably, the shield has dimensions comparable to or larger than the tower diameter, or the diameter of a relevant tower segment.

During higher than preferred ambient wind conditions, the shield may thereby create a certain lee in which a tower may stand. In this way, oscillation of a wind turbine, or tower under construction may be prevented, interrupted or reduced. In particular, construction of a tower may continue, thereby avoiding possible delay or damage, and lowering associated costs.

The method may be implemented when local wind conditions are below or above a nominal wind limit considered to pose a significant risk of oscillation. This wind limit may vary according to the tower parameters such as e.g. its height. In aspects, a wind shield may be temporarily held at relevant location proximate a tower or part thereof. In aspects, a wind shield may be transported to a wind turbine site prior to a relevant construction operation. In aspects, a wind shield may be deployed at a relevant site if or when wind speed conditions appear to be approaching or exceeding a nominal limit value for causing oscillations. In aspects, wind shield may be deployed at a relevant site if or when tower oscillation is detected. Preferably, the wind shield may be movable. The wind shield may thereby be transported to or from a construction or service site. The shield may preferably be manipulated i.e. steered into position proximate a tower under conditions likely to lead to oscillation of the tower or when such oscillation is detected. The shield may be manipulated, i.e. steered away from a tower, in particular if a phase of oscillation falls below an acceptable limit.

Preferably, a wind shield may be controlled such that it may be brought close enough to a tower or tower segment or tower stack under construction, to prevent, interrupt or reduce oscillation. In some cases, the wind shield may be brought into contact with the tower or part thereof.

Preferably, the wind shield may be an aerodynamic body capable of flight or tethered flight. Preferably, the shield may have a continuous or substantially continuous surface capable of obstructing air flow when held against a flow of air. Preferably the wind shield may be collapsible.

Preferably, the shield may be held aloft using tethered cables. Preferably, one or more tethered cables may co-operate with a winch. Preferably a winch associated with a tethering cable may be a powered winch, preferably an electric motorised winch. A tethered wind shield may be held aloft by virtue of the effect of the wind on its aerodynamic shape.

Preferably the method is implemented using a shield which is in flight.

Preferably, a wind shield may be steered into position by controlling tethered cables. Hence, tethering cables may be control cables. Tethering cables may include power cables and brake cables. Some cables may be emergency guy cables. Some or all tethering cables may be on winches. A winch may include a powered rotary drive. Winches may be associated with a control system. Winches may be comprised in a winching module. A winch module may be restrained by ballast. For example, a winch module may be a truck or container or construction comprising one or more winches and optionally a ballast element.

A winching module may include or be associated with a control console. A control console may be part of a control system. A control system may be an automatic or semi-automatic control system. A winch module comprising winches may be fixed to the ground. A winch module comprising winches may be fixed to ground-based mobile platforms. Winches or winch modules may be carried on a truck bed.

Shield control cables may be held aloft using a ground-based structure such as a tall structure such as a mast or a crane. A winch module associated with these control cables may be on ground, with control cables supported via an intermediate sheave or block. Such an intermediate sheave or block may be held aloft by a crane or other ground-based structure. Alternatively, a winch module may be held aloft, suspended from a crane or other tall structure.

The wind shield may be a power kite. For example, it may be a rigid power kite with struts, or partially rigid, including some struts. An example of a rigid kite may be a delta kite or lozenge shaped kite or any other common or suitable shape. Alternatively, the shield may be a semi-rigid kite, such as a closed cell kite, with no struts or with few struts. Still alternatively, the shield may be a non-rigid kite such as an open cell kite with no struts. Preferably, the shield may be self-launchable, such a self-launchable kite. The kite, whether rigid, semi-rigid or non-rigid may in particular comprise a canopy. The canopy may be of a wind-proof material. In particular the canopy may be in the form of a sail cloth or sail sheet or other wind-proof material sheet. The purpose of the shield is to disrupt or block the flow of air carried onto it by the wind. In particular, the shield shall disrupt or block air flow towards a wind turbine or its tower, whether fully or partially constructed.

Semi-automatic control of the shield may be carried out using control wires wound on motorised winches, with winch-control, and thereby control of the shield in flight being controlled by an operator. To this end, an operator may use a control console. In automatic control of the shield, the winch control for controlling the shield may be automated for example using software and a control system linked to the winches.

Preferably, the shield may have a maximum dimension comparable to or larger than at least one cross-dimension diameter of the part tower being shielded. Preferably, during the method of the invention, a position of the shield proximate a tower segment may include a minimum distance between said shield and said tower, comparable to maximum diameter of the tower. Preferably, during the method of the invention, the shield may be held sufficiently proximate the tower to create a full or partial wind shadow at the tower. Preferably, the proposed method is operated such that it acts to reduce the wind force on the tower, enough to prevent or interrupt oscillations thereof, or to reduce oscillations to a benign level. That is to say that by means of this method, and while the method is being implemented in respect of a relevant tower, the wind force acting on the tower is momentarily less than the wind force which would act on the tower in the absence of said shield.

In aspects, there may also be a method of shielding a wind turbine at a wind turbine site, the method including: providing a shield; providing manipulation equipment associated with said shield; and the method further including holding said shield, preferably in flight, proximate and upwind of said wind turbine, said shield being held aloft by means of said manipulation equipment. Said method may be performed while said wind turbine is de-powered. Or said method may be performed during wind induced oscillation of said wind turbine.

The proposed shielding method for preventing tower oscillation is defined in appended claim 1 . Further preferred features of the method are defined in subclaims 2-20. A relevant apparatus is defined in appended claim 21. Further preferred features are defined in subclaims 22-24.

Some aspects and features of the method and equipment will be explained by way of non limiting example with reference to appended drawings, in which:

Fig. 1 shows a not-to-scale example of a shield in use at a wind turbine construction site;

Fig. 2 shows a not-to-scale alternative example of a shield in use at a wind turbine service site; and

Fig. 3 shows a not-to-scale example of a shield in use during oscillation of a wind turbine tower.

Fig. 4 shows another not-to-scale example of a shield in use during wind turbine tower oscillation.

Fig. 5 shows a not-to-scale example of a shield in use during wind turbine construction.

Fig. 6 shows a schematic not-to-scale diagram of aspects of exemplary equipment for implementing aspects of the method.

In Fig. 1 there is illustrated a wind turbine construction site 1 , including a partially

constructed wind turbine 50. In the illustration of Fig. 1 , the partially constructed wind turbine 50 which is shown includes a tower 56, a nacelle 57, a hub 59, and a partially constructed rotor 51. The example illustrates a wind turbine blade 55 being lifted to a partially constructed rotor 51 . In the example shown the wind turbine part 50 is under construction and therefore de-powered. Any anti-oscillation systems in the nacelle 57 or in the tower 56 may be inactive. This may make the wind turbine 50 susceptible to oscillations under certain wind conditions. The lifting apparatus 60 illustrated is a crane, specifically a crawler crane, although other types of crane or lifting apparatus 60 may be used. Momentary, ambient wind speed W at the site 1 is illustrated by a solid directional arrow. During wind turbine construction or servicing, when ambient wind conditions 1/1/ exceed a nominal limit wind speed, a risk of oscillation may exceed acceptable levels. Oscillation of a tower 56 may lead to an interruption, cessation or delay of construction and may lead to serious damage of the tower itself, possibly necessitating replacing one or more segments thereof or the entire tower. According to aspects of the invention, when wind speeds reach a level above a critical oscillation risk level, of if oscillations are detected, there may be initiated a method of reducing the wind speed at the tower 56. A wind shield 40 may be deployed and held up close enough to the tower 56 to create a wind shadow at the tower 56, enough to prevent or disrupt wind flow around the tower 56, to an extent sufficient to prevent or reduce oscillations. In particular, the shield 40 may reduce the wind speed at the tower 56 to a lower level w, indicated by a smaller solid arrow in the Figures. The wind shadow can be maintained by bringing and maintaining a shield 40 within a distance d proximate the tower 56 or segment 56a thereof. A closer proximity to the tower 56 provides a more effective shadow. For risk mitigation reasons, it may be preferred to adjust the operating distance d between the shield 40 and the tower 56 depending on the dimensions of the tower 56. In the case illustrated, the operating distance d between the tower 56 and the shield 40 may be of the order of the local diameter of the tower 56. In embodiments, the minimum distance d may be not less half the tower 56 diameter. In other aspects, the distance d may be adjusted to a different value depending on the size of the shield 40: a larger shield 40 may adopt a greater distance d. Alternatively, in aspects, the distance d may be zero, in particular in case it is desired to land the shield 40 on the tower: that is to say, to bring the shield 40 into contact with the tower 56. This technique may increase the damping effect of the shield 40 on the tower 56 and may enhance the interruption or reduction of tower 56 oscillations.

The shield 40 illustrated in Fig. 1 may comprise a substantially continuous surface in the form of a canopy. As illustrated, the shield 40 may be a power kite. The power kite may be open-celled and non-rigid or closed-celled and partially rigid or semi-rigid. A semi-rigid power kite may be non-rigid when collapsed and partially rigid when deployed. In other aspects, the shield 40 may comprise a kite with struts. In some aspects, a power kite may be partially rigid and including struts. When required, a shield 40 may be manually deployed from the ground and steered into position proximate a tower 56 being shielded. As long as ambient wind speeds W are sustained above or near the predefined nominal limit wind speed, the shield 40 may remain deployed and held proximate a tower 56, thereby enabling e.g. a construction of the tower 56 or servicing operation of the wind turbine 50 to continue uninterrupted. If the ambient wind speed l/l falls to below the nominal limit wind speed considered to pose an oscillation risk, and if it remains below the nominal limit wind speed for a sustained period of time, the shield 40 may be collapsed and retracted. A decision to begin or to cease operating the shield 40 may also be based partly or fully on local meteorological forecast information. Alternatively, a decision to begin or to cease operating the shield 40 may also be based partly or fully on detected oscillations at a tower 56. Oscillations may be detected visually by personnel. Alternatively a tower 56 or tower segment 56a may be fitted with an oscillation detector 83. In embodiments, an oscillation detector may be a camera (not shown). In embodiments, an oscillation-detection camera may be positioned nearby one or more towers 56.

Still referring to Fig. 1 , a shield may be held aloft by tethered cables 44, 42. In particular, in preferred aspects, the tethered cables may be relayed to one or more winches. As shown in Fig. 1 , the shield 40 may be a conventional kite such as a power kite, capable of being controlled and steered by wires in the manner of any kite or power kite. In particular, a shield 40 may have a trailing edge, and a leading edge, connected by a canopy. A bridle 22 connected to guide cables 42, 44 connects a canopy of the shield 40 with the anchoring and steering arrangements for the shield 40, such as winches 28, 29 and a control system if applicable. In Fig. 1 , a manipulating apparatus 45 may comprise a bridle 22 associated with cables 42, 44 and a winching module 26 which performs both functions of anchoring the shield 40 and controlling its flight. Advantageously, the bridle 22 may be attached to a steering system including power cables 42 and a brake system including brake cables 44. When both sets of cables 42, 44 are held in tension, the shield 40 may be operational to be held proximate a tower 56 and effective to partially or fully block wind W. For depowering a shield 40, one or more brake cables 44 may be slackened to increase air escape out from the trailing edge of the shield 40. Conversely, in order to re-power the shield 40 in the wind, the brake cables 44 may be taken up, i.e. tension may be applied to them e.g. by pulling on them or winding them in. If required, a shield 40 may be abruptly collapsed by sudden release of the brake cables 44. Steering left or right may be carried out by selectively drawing in a corresponding right or left power cable 42. In the example shown, the power cables 42 at a respective port and a starboard side of the shield 40 are each relayed to a respective steering winch 28. In the example shown, the brake cables 44 at a respective port and a starboard side of the shield 40 are all relayed to a brake winch 29. Other configurations may be chosen, in keeping with e.g. known kite control systems. In the example of Fig. 1 , each winch 28, 29 is preferably independently controllable and constitutes an anchor point for a respective cable. Each winch also constitutes an element of a shield control mechanism and thereby forms part of the shield’s manipulating apparatus 45.

The winches 28, 29 are preferably received in a winch module 26 which, in the case illustrated is placed fixed atop a mobile platform 20, shown by way of example as a bed-type truck. The winch module 26 may advantageously include a ballast system. The ballast system may comprise a ballast fitment such as e.g. a tank for adding liquid or fluid ballast such as water or sand. Or the ballast system may e.g. include an appropriate amount of a heavy material. In any event, the ballast system may be capable of stabilising the shield 40, such that it can maintain and sustain a steady position in flight, in particular, proximate to a tower 56.

In Fig. 2, there is illustrated an alternative example, in which the winching module 26 may be positioned supported on the ground, while the control wires 42, 45 may be relayed over a pulley-block 24. The pulley block may be suspended from or atop a second lifting apparatus 66 shown by way of example in the form of a mobile telescopic boom crane. In this example, the range of flight of the shield 40 may be relatively restricted in view of the short length of the free control wires 42, 44 between the pulley block 24 and the shield 40. With the free control wires 42, 44 running from close to ground level as per the example in Fig. 1 , the flight range of the shield 40 may relatively greater. For some applications, a more restricted movement range of the shield 40 may be desirable and may ease overall positional control in relation to the tower 56 being shielded. In aspects, the pulley block 24 may be secured against swinging action or against rotation using taglines (not shown) running between the pulley block 24 and an anchor, preferably at ground level.

In Fig. 3, a still further example is shown in which the winch module 26 may be suspended from or atop a second lifting apparatus 66 at a wind turbine service site 2. In this example, a service operation is being carried out at a wind turbine 50, including lifting a component 52 to or from the nacelle 57. During such operations, the nacelle may be de-powered. Active oscillation-damping systems may also be de-powered. In this example, also a ballast included in the winch module 26 is suspended from the secondary lifting apparatus 66, as part of the winch module. This gives stability to the winch shield 40. Optionally, the examples of Figs. 2 and 3 may be most effectively implemented when shielding tower parts 56a which are relatively higher up the tower 56. In Fig. 3, the tower part 56a being shielded may be an upper tower segment. The exemplary lift is being carried out by means of a lifting apparatus 60 in the form of a nacelle crane. A nacelle winch or other lifting apparatus 60 could equally well be used. Also visible in Fig. 3 is a tag line 29 running between the suspended winch module 26 and a ground anchor, shown here by way of a mobile platform 20, such as a truck.

In further aspects, as illustrated for example in Fig. 6, the manipulating apparatus 45 may include a control system 77, in particular a microprocessor type control system 77 including memory registers. The control system 77 may optionally be comprised by a part of a winch module 26 or may be otherwise associated with a winch module 26. It may be semi- automatic, and may comprise for example a control console (not shown) to be operated by an operator. The control console may include at least a control actuator for steering, and a control actuator for increasing or decreasing air escape at the shield 40. Steering control may be used predominantly for adjusting port or starboard positioning or travel direction of the shield 40. Air escape control may predominantly adjust height of the shield 40 or its speed of motion. A steering actuator may in particular be associated with a drive motor at a steering winch 28 or a set of powered steering winches 28. An air escape actuator, or brake actuator may be associated with a drive motor at a brake winch 29 or a set of powered brake winches 29. In this way an operator may manipulate the shield 40 into position and maintain it in the desired position. If required, the shield 40 may be easily steered to follow the contour or shape of a tower 56 while it is being shielded. If required, the shield 40 may be easily steered to maintain its position at a tower 56 such as a partially constructed tower 56, e.g. comprising one or more tower segment 56a. If required, and where the relevant tower 56 is larger than the shield 40, the shield 40 may be easily steered to follow the shape of the tower 56. Optionally, the shield 40 may be held aloft at or near an upper region of a tower 56. This is the place when the greatest effect due to vortex-shedding may occur. Preferably the control console may include an emergency, automatic collapse and retraction control switch.

In a further aspect, the control system 77 may comprise a fully automatic system including shield control software 88 for steering the shield 40 to port or starboard (left or right) and for raising or lowering the shield 40. The control system 77 may include ambient wind speed \N detection or input apparatus or may otherwise receive regular ambient wind speed transmissions from a wind speed information source 81 such as a data source or sensors or a forecasting source. Alternatively, the control system 77 may automatically receive an input from an oscillation sensor 83, e.g. at a tower 56 or tower segment 56a.

For improved control of the shield 40, according to any aspects of the method and equipment disclosed herein, there may be provided position indicators 74, 72. These are illustrated for example in Fig. 6 or Fig. 5 or Fig. 2. A respective position indicator 74 may be provided at a tower 56. A position indicator 72 may additionally or alternatively be provided at the shield 40. These position indicators 74, 72 may be transponders such as for example RFID tags or GPS receiver-emitters or any other position-indicating emitter device. These may communicate with one or more dedicated receiver 76, 78 at the control system 77. A detection system 82 associated with the control system 77 may monitor in real time the respective positions of a shield 40 or/and a tower 56. In aspects, there may be more than one position indicator 76 at a tower 56. For example, two or more tower segments 56a may each comprise a position indicator 74. Optionally, each segment of a tower 56, or partially constructed tower 56 may include a position indicator 74. Any segment 56a may include one or two or more position indicators 74. In this way, more than one shield 40 may be flown in proximity to a relevant one of the tower’s multiple position indicators 74. In effect, in case multiple wind shields 40 are used together at a tower 56, then each shield 40 with its position indicator 72 may be paired with a respective one indicator 74 from among multiple indicators 74 at the relevant tower 56 or tower segment 56a.

The control system 77 may additionally or alternatively receive information or monitored values representing wind direction and/or wind speed from wind sensors 79, 80 which sensors 79, 80 may form part of the control system 77 or manipulating apparatus 45.

Optionally, a wind speed sensor 79, 80 may be positioned at a tower 56 or at a lifting apparatus such as a crane 60. A wind sensor 79, 80 may transmit its sensed values to the control system 77. Optionally, a tower 56 or a part therefor or a tower segment 56a may be fitted with both a wind speed sensor 80 and a position indicator 74. Optionally, a wind speed sensor 79, 80 may be positioned for local, ambient wind speed measurement, such as at a lifting apparatus 60, e.g. on an exposed crane boom section or exposed jib section. This is illustrated by way of example in Fig. 5. By way of example, a wind speed sensor 79 may be positioned on a lifting apparatus 60 in use for lifting or handling a relevant tower segment 56a or wind turbine part during construction of a wind turbine.

The control system 77 may receive additional data input information e.g. from an operator, via an input interface 85 if required. This may include input information designating relevant towers 56 or tower parts 56 to be monitored and possibly shielded. Thus, an operator may input a tower 56 or tower segment 56a, in particular, by entering an identification designation of a position indicator 74 at the tower/segment 56/56a. An operator may input a shield 40, in particular by entering an identification designation of a position indicator 72 at the shield. An operator may also enter an indication of which wind sensors 79, 80 or data sources 81 are to be used as input value sources for a shielding operation at a given tower 56 or tower part or tower segment 56a. The control system 77 may automatically or in response to an emitted interrogation signal, receive feeds of respective sensory measurements, or data inputs representing weather and/or positional information of the relevant tower/segment 56/56a and shield 40. Input information via interface 85 may for example include wind limit thresholds such as nominal limit values, or threshold values for oscillations detected e.g. by a sensor 83. Input information may further include maximum and minimum values for wind speed and/or oscillation levels and/or required proximity d to be maintained between a relevant tower/segment 56/56a and the shield 40. A maximum threshold value for tower oscillation may designate a threshold value for oscillation, which, if detected, e.g. by an oscillation sensor 83, may trigger automatic deployment of a shield 40. A minimum threshold value for tower oscillation may designate a threshold value for oscillation, which, if detected, e.g. by an oscillation sensor 83, may trigger automatic ceasing of a shielding operation. Signals to deploy a shielding operation or to cease such an operation may also be entered manually e.g. via an interface 85.

In aspects, the controller 77 may call up predetermined data from a databank including data such as records of wind turbine towers 56, with their corresponding, known oscillation characteristics, such as wind speeds at which oscillation may be induced. For any tower type 56 wind speed and oscillation data may be stored for each set of respective heights which such a partially constructed tower may have. E.g. with a first segment 56a only, or with a first and a second segment 56a, or with a first, second and third segment, 56a etc., including the full tower height. The file for a given tower may also include the required maximum or minimum proximity distance c/from the shield 40, or a recommended proximity d. As mentioned previously, there may be circumstances in which it is recommended or necessary to bring a shield 40 into contact with a tower 56 during a shielding operation, to increase the damping effect, e.g. if the tower 56 is undergoing oscillation. Equally, a data file may include look up tables for different shields 40 which may be used with the manipulating apparatus 45, including proximity data relevant for the particular selection of shields available. A user may simply input a shield 40 type to be used and a tower 56 to be shielded from wind, perhaps including the tower height, following which the system 77 will automatically generate and/or automatically apply applicable predetermined wind speed limit values and/or detected oscillation values for collapsing and retracting a wind shield 40. The system 77 may thereafter operate automatically, launching a shield 40 when required, based on input weather or wind data and controlling the steering winches 28 and braking winches 29 as required, for the maintaining the required position or flight of the shield or shields 40 in relation to the tower 56 being shielded.

In further aspects indicated in Fig. 6, a position indicator 72 provided at a shield 40 may transmit signal information to a receiver associated with the position indicator 74 at a tower 56 being shielded or requiring to be shielded, such as a fully or partially constructed tower 56. In this way, the position indicator 74, and an associated receiver, may constitute or be operated as a proximity sensor. Preferably, according to this aspect, the position indicator 74 may transmit proximity data to said control unit 77, representing a distance between said shield 40 and said tower 56. Conversely, a position indicator 72 provided at a shield 40 may transmit signal information to a receiver associated with the position indicator 72 at a shield 40. In this way, the position indicator 72, and an associated receiver, may constitute or be operated as a proximity sensor. Preferably, according to this aspect, the position indicator 72 may transmit proximity data to said control unit 77, representing a distance between said tower 56 to be shielded from wind, and said shield 40. In other words, the shield 40 and tower 56 may comprise transmitter and receiver elements configured to indicate a separation between the two. The method may include transmitting from a position-indicator 72 or 74, a signal indicative of a separation distance between said shield 40 and said tower 56, to said a control system receiver 78, 76. In effect, the distance d between a position sensor 72 at a shield 40 and a position sensor 74 at a tower 56 may be used as a proxy value for the separation between a tower 56 and a shield 40.

In aspects, a warning alarm may be triggered if the control system 77 detects that the tower 56 being shielded from wind and the shield 40 are too close together, e.g. below a predetermined absolute minimum distance y apart. The warning alarm may sound in either or both a semiautomatic, operator controlled system or method, or in a fully automatically controlled method or apparatus. This alarm may be overridden in case it is attempted to bring the shield 40 into direct contact against a tower 56.

In further aspects, the shield manipulation system 45 may be operated in a semi-automatic mode by selecting a target value for the separation distance d between the shield 40 to be used and the relevant tower 56 being shielded. This may be done automatically using a control system 77 output based on input data, e.g. via an input interface 85, for the relevant tower type and the relevant shield type 40 and based either on the control system’s calculation or on stored look-up type data, or both. A target value for the separation distance d may also be selected manually and input by a user, e.g. via input interface 85. Hence, a separation distance d may be adopted and referred to as a target separation distance. The control system 77 may select and apply a tolerance range T(+) and T(-) above and below the target value for the separation distance d, or this may be selected and input manually. For example, a tolerance range of 10% or 15% or 20% or more above and below the target value for the separation distance d may be selected. Other values may be selected as appropriate for a given set of circumstances. For example, a set of tolerance values may be chosen with a higher upper tolerance range T(+) than the lower tolerance range T(-). This may for example allow the shield 40 to be held and maintained in operation close to a recommended minimum separation distance d. In aspects, a target value for the separation distance d may be chosen to correspond with a minimum recommended operating separation distance for a given tower 56 and shield 40 combination. The operator may then manipulate the shield 40, once aloft, so that it stays at or close to a selected target value for the separation distance c/from the tower 56 being shielded, e.g. in particular, a tower 56 under construction. This may be achieved using controls associated with winches 28, 29 for the guide wires 42, 44. In aspects, the control system 77 may sound a control alarm signal if the upper tolerance limit T(+) is reached during operation of the shield 40, for example a high pitched audio signal or a high frequency vibration of a user’s operating console. This may prompt the user to control the shield 40 to move further towards the target value for the separation distance d in relation to the tower 56 being shielded i.e. towards the tower 56. The control system 77 may sound a control alarm signal if the lower tolerance limit T(-) is reached during operation of the shield 40, for example a low pitched audio signal or a lower frequency vibration of an operating console. This may prompt the user to control the shield 40 to move further towards the target value for the separation distance d in relation to the tower 56 being shielded from wind, i.e. away from the tower 56. Steering control alarms may be distinct and distinguishable from previously mentioned warning alarms. In this context, a lower tolerance value T(-) for the separation distance d may be distinct from an absolute minimum limit distance y. In particular, target values for the separation distance d and associated tolerance limits T(+) and T(-) may be used for steering and control purposes of the shield. Whereas an absolute minimum limit distance y may be applied as an additional safety measure, possibly in associated with an instruction to cease operations or with an automatic collapsing mode of the control system 77.

In a still further aspect, the control system 77 may operate in a fully automatic shield 40 control mode as discussed above, using an internal target value for the separation distance d and applying upper and lower tolerance values T(+) and T(-) to gain steering flexibility for maintaining the shield 40 as close as possible to the target separation distance d. In examples, preferably, the shield 40 may have a maximum dimension s comparable to or larger than the maximum dimension p of the tower 56 to be shielded from wind. Where the tower 56 to be shielded is a tower segment 56a, the shield 40 may have a maximum dimension s larger than the maximum chord dimension of the segment 56a.

In a further aspect, a control mode of the control system 77 may additionally determine a difference value between a sensed local wind condition Wfrom a sensor 79 and a sensed value w from a sensor 80 at a tower 56 to be shielded. The size of the difference value may be used to adjust and maintain the shield 40 position towards or away from the tower 56. In particular, if a determined difference value between a sensed local wind condition Wand a sensed value i/i/from a sensor 80 at a tower 56 being shielded is larger than a target difference value, the control system 77 may to adjust and maintain the shield 40 position closer to the tower 56. If a determined difference value between a sensed local wind condition Wand a sensed value i/i/from a sensor 80 at a tower 56 being shielded is lower than a target difference value, the control system 77 may to adjust and maintain the shield 40 position further away from the tower 56. Alternatively, if a determined oscillation value sensed by an oscillation sensor 83 at a tower part indicates that oscillations are detected above a minimum permitted threshold value, the control system 77 may adjust and maintain the shield 40 position closer to the tower 56. Alternatively, if a determined oscillation value sensed by an oscillation sensor 83 at a tower part indicates that oscillations are detected significantly above a minimum permitted threshold value, the control system 77 may emit an alarm signal. In these circumstances, the control system 77 may trigger the launch of an additional shield 40 to be flown nearby another part of the tower 56 where more significant oscillation has been detected.

Preferably, during aspects of the method proposed herein, a position of the shield 40 proximate said tower 56 may include a minimum distance d between said shield 40 and said tower 56 equal to or greater than three times a maximum dimension s of said shield 40. Still preferably, a position of the shield 40 proximate said tower 56 may include a distance d between said shield 40 and said tower 56 equal to or geater than two times a maximum dimension s of said shield 40. Optionally, a position of the shield 40 proximate said tower 56 may include a minimum distance d between said shield 40 and said tower 56 equal to a maximum dimension s of said shield 40. Optionally, a position of the shield 40 proximate said tower 56 may include a minimum distance d between said shield 40 and said tower 56 equal to or greater than three or two times a maximum diameter dimension p of said tower 56 (or tower segment 56a). Still preferably, a position of the shield 40 proximate said tower 56 may include a minimum distance d between said shield 40 and said tower 56 equal to or greater than a maximum diameter dimension p of said tower 56 (or tower segment 56a). A tower segment referred to herein may be a tower portion 56a. In the present specification, a separation distance d may be adopted and referred to as a target separation distance.

As already mentioned, when an object, especially an elongate object, is exposed to wind flowing around it, an effect known as vortex-shedding can occur, causing the elongate object to oscillate. In the case of a wind turbine 50, some of its elements can be moved in relation to the wind. For example, a nacelle yaw drive may be actuated to change the orientation of a wind turbine nacelle and rotor, in relation to the prevailing wind. Or wind turbine blades 55 may be pitched about their own axis using a pitch drive at the rotor hub 59, in order to vary the angle of attack which the blades 55 present to the prevailing wind. These adjustments can help to prevent vortex-shedding, for example during a period of inactivity of a wind turbine 50, possibly during a period of unusually high wind at the turbine 50. However, a problem may arise when a wind turbine 50 is de-powered, which is to say when a wind turbine has no power to its own sub-systems. Therefore, in a further aspect, the method of using a shield 40 for providing wind shadow may be used at a wind turbine 50 when it is de- powered, in particular for preventing vortex-shedding. In Fig. 4, there is shown a simplified diagram of alternative aspects of the invention, with some details omitted for clarity. In particular, the invention may be deployed at an already constructed wind turbine 50 when performing service activities on the wind turbine 50. Such activities may include deploying service equipment or service personnel, e.g. more or less locally, at external regions of the tower 56, the nacelle 57, or the blades 55, or hub 59. In Fig. 4 a free-standing tower 56 is shown in a state of oscillation induced by the effect of the ambient wind passing around it at a speed W. The tower oscillation is indicated by large arrows marked Os. Such oscillations Os will generally abate when the wind speed 1/1/drops. Since high ambient wind speeds W can prevail for prolonged periods, the effects of oscillation on a part 58 of a wind turbine can have a significant impact on the fatigue life of that part 58, such as e.g. on a tower 56. In some cases, the positioning of a nacelle 57 atop a tower 56 of a partially constructed wind-turbine 50 can prevent vortex-shedding induced oscillations, by creating a damping effect on the tower. I.e. by virtue of the significant additional mass of the nacelle 57, the resonant frequency characteristics of a tower 56 or mast may be significantly changed. Alternatively, a nacelle 57 or a finished tower 56 may include oscillation-damping systems within them, to counter such oscillations. Nevertheless, a wind turbine 50 may encounter oscillations sometimes, perhaps if an oscillation-damping system is inactive or defective. Or perhaps because a turbine 50 is periodically at a standstill. Or perhaps because a wind turbine 50 is de-powered and thereby unable to make e.g. yaw or pitch adjustments to the nacelle or blades, or unable to activate an active oscillation-damping system. If oscillations Os are encountered, they can be difficult to stop. The shielding of a tower 56 of a wind turbine 50 from wind using a shield 40 may place the relevant tower 56 or part thereof in a wind shadow or partial wind shadow, where wind speed w is reduced in relation to ambient wind speed W. This in turn may stop, prevent, disrupt or interrupt the oscillation-inducing effect of wind on a tower 56, thereby preventing or stopping or interrupting oscillations. The distance d between a shield 40 and a relevant tower 56 part or tower segment 56a may be such as to impart a wind shadow with speed w at the relevant tower 56, while keeping a safe distance from the tower 56. Preferably, it may be desirable to prevent a shield 40 from contacting the tower 56. A shorter separation distance d between a shield 40 and a tower 56 may allow for increased effectiveness of the shield 40 being used.

As already mentioned, in aspects, the control system 77 may be associated with one or more oscillation sensor 83 (shown by way of example in Figs. 6 and 5), preferably positioned at a tower 56 or tower segment 56a. An oscillation sensor 83 may communicate information with the control system 77. An oscillation sensor 83 may output a signal representative of sensed oscillatory motion at a relevant tower segment 56a. An oscillation sensor 83 may indicate a sensed value upon interrogation from a control system 77. Alternatively, an oscillation sensor 83 may periodically emit sensed values to the control system 77. A warning signal may be generated from the control system 77 when an input value received in the control system 77 from an oscillation sensor 83 is above a warning threshold level. An alarm signal may be generated from the control system 77 when an input value received in the control system 77 from an oscillation sensor 83 is above an alarm threshold level.

Respective alarm or warning threshold levels may be input into a control system 77 using an input device 85. An alarm threshold value may indicate that a tower 56 has attained a level of oscillation which signals a possible need for action. A warning threshold value may indicate that a tower 56 has attained a level of oscillation which signals an imminent need for action. The determination that a warning or alarm signal level is reached may depend on combined factors of oscillation level and time duration of oscillations.

In addition, a position indicator 74 may be provided at a tower segment 56a of a tower 56. In aspects, a tower 56 may comprise multiple position indicators 74. For example, two or more segments 56a of a tower 56 may each comprise a position indicator 74. The need for more than one position indicator 74 at a tower 56 may arise in case more than one shield 40 is required for providing wind shielding to it. Thus, any wind shielding method set forth herein may be performed using two or more shields 40 simultaneously.

Fig. 5 shows further aspects of the invention, wherein in particular, a shield 40 is used to shield a wind turbine tower 56 from oscillation-inducing effects of the ambient wind. Fig. 5 illustrates a wind turbine 50 under construction. During the construction, a fully or partially constructed tower 56 may be subject to the effects of ambient wind. In particular, a fully or partially constructed tower 56 may begin to oscillate as indicated by arrows, labelled Os.

One or more shields 40 may be deployed to counter the effects of wind on the tower 56 and to bring the tower 56 into a wind shadow with a lower wind speed w. This speed w may be known as the wind shadow speed. To this end, the control system 77 may receive position signals from position indicator 72 at a shield 40 and from a position indicator 74 at a tower 56. The shield 40 may thereby be moved into a wind-shielding position with the shield 40 at an appropriate, known distance from the tower 56. In embodiments, more than one tower segment 56a at a tower 56 may be provided with a position indicator 74. This may allow a single shield 40 to be flown close to either of these, or periodically, to fly from a position near a first position indicator 74 to a position near a second position indicator 74 at a same tower. Alternatively, more than one shield 40 may be flown simultaneously, with each shield 40 being held at a wind-shieling position near a respective tower segment 56a. Deployment of a shield 40 at a tower 56, to stop oscillation may be initiated by an alarm or warning signal from a control system 77, as a result of inputs, possibly from an operator or possibly from an oscillation sensor 83. Once oscillation has been interrupted, reduced, brought under control or eliminated, a decision may be taken whether to cease the shielding operation or whether to maintain it as a preventive measure. For example, if tower oscillation is interrupted for long enough to install a nacelle 57 atop the tower 56, then a wind-shielding operation may be stopped, on the presumption that the tower’s susceptibility to wind-induced oscillation may thereby be sufficiently diminished. In particular, the placement of a nacelle 57 atop the tower 56 may in itself be enough to bring an end to the tendency for the tower 56 to oscillate. With the tower oscillation problem stopped, prevented or interrupted, by the effect of the shield 40, the construction process may continue. Further improvement in oscillation prevention may be established when the wind turbine’s own systems are no longer de- powered, i.e. powered up. The wind turbine’s own systems may include active or passive oscillation damping systems.

Figures 1 , 4 and 5 all illustrate the deployment of more than one shield 40 at a single wind turbine tower 56. The illustrations show two shields 40 being simultaneously deployed, although more than two shields may be deployed if needed. Each shield may have a position sensor 72, while each tower 56 may have two or more position sensors 74 spaced apart along the length of the tower 56. In particular, each segment 56a of a tower may have a position sensor 74. A control system 77 may pair the position sensors 72 of each respective kite 40 with a respective one position sensor 74 at a given tower 56. This will allow the control system to keep the shields 40 in proximity, by a distance d, to a respective part of the tower 56, without the shields 40 interfering with each other. Preferably therefore, position sensors 74 at a tower 56 may be spaced apart by more than a maximum dimension s of a shield, preferably by more than twice the maximum dimension s of a shield. 40. The effect here is to increase the shielding effect on the tower 56, to reduce, as much as possible, any oscillations it is undergoing.

A method of preventing wind turbine tower oscillation may include:

Monitoring wind speed at a wind turbine site; optionally comparing measured wind speed data with database information relating to wind speeds which may induce tower oscillations; determining whether measured ambient wind speeds correspond to wind speeds which may trigger tower oscillations; generating an alarm signal via a control system 77, and/or automatically deploying and flying a shield 40 proximate to a tower 56. The shield 40 may be automatically or manually collapsed if wind detected speeds drop below a threshold corresponding to a low risk of tower oscillation, in particular, if wind detected speeds drop below a threshold corresponding to a low risk of tower oscillation for a period longer than a predetermined minimum time period.

A further method of stopping, interrupting or reducing wind turbine tower oscillation may include: Monitoring tower oscillations at a fully or partially constructed wind turbine tower 56 by means of oscillation sensor 83 at a tower or using camera or operator information;

comparing measured oscillations with a threshold for oscillation prevention; generating an alarm signal via a control system 77, and/or automatically deploying and flying a shield 40 proximate to a tower 56. Optionally, the method may include flying a shield 40 such that it contacts a tower 56. The shield 40 may be automatically or manually collapsed if detected oscillations fall below a minimum threshold level. Preferably, the shield 40 may be collapsed if detected oscillations fall below a minimum threshold level for a predefined minimum time period. Preferably, the shield 40 may be collapsed if detected oscillations fall below a minimum threshold level and if detected wind speeds fall below a minimum threshold level, preferably for a predefined minimum time period.

Features disclosed or claimed in this specification may be combined together in any way appropriate. Possible combinations of features disclosed herein, may be limited only by the laws of physics or by combinations which may be manifestly impossible for other reasons.

A tethering cable may take the form of a tethering line or control line 42, 44 or guide line 42 or steering line 43 or other tag line 29. The term“local wind” is used to designate the ambient wind 1/1/ at a given object or location 1 , 2. Ambient wind 1/1/ may be a momentarily prevailing wind condition, in particular a wind speed or wind direction or both. A full or partial wind shadow may be understood as reduction in ambient wind force at a particular location or object. A reduced wind speed w may in particular imply a reduced wind force incident on a relevant object. In the present context, a maximum dimension of a shield 40 may be measured across a canopy region of said shield 40. Preferably, a maximum dimension may be measured in a laid-out flat condition of the shield 40; for example from corner to corner.

A reference to a tower being shielded by said shield may be a reference to a tower or part thereof.